1. Field of the Invention
The present invention relates to power supplies and more particularly to circuitry adapted for use in conventional power supplies of the pulse width modulated (p.w.m.) type to provide for frequency synchronization.
2. Description of the Prior Art
Many types of power supplies now make use of the well-known pulse width modulation (p.w.m.) technique to provide a signal to drive one or more power switching devices at a relatively high frequency. The p.w.m. technique makes use of a sawtooth waveform of predetermined frequency usually in the order of 20 kilohertz for purposes of generating the drive signal to the power switching devices. While the p.w.m. circuitry may be embodied by using any one of the circuit configurations well known in the art, it has become increasingly common to use devices such as integrated circuit chips to embody the p.w.m. circuitry. These chips ordinarily include an internal clock which provides the sawtooth waveform.
The supply which uses the p.w.m. technique is ordinarily connected to a load to provide power thereto. The load may, for example, be a computer controlled device which also has its own internal clock. The computer clock has a frequency which is typically in the order of several megahertz. Quite often the computer clock may be divided down in order to provide a variety of clock frequencies to the circuits contained therein. Ordinarily the internal clock of the p.w.m. chip is not at a frequency which is exactly the same as the frequency of the nearest whole number submultiple of the computer clock. This whole number submultiple will be referred to hereinafter as the master clock. Having different frequencies for the master clock and the p.w.m. internal clock gives rise to signal components having frequencies which represent the sum and difference of those frequencies. These components then appear as noise within the computer and may interfere with the operation thereof. It is, therefore, desirable that the clock frequency of the p.w.m. chip be synchronized with the frequency of the master clock of the load and thereby minimize the occurrence of any such noise.